The present invention relates to a limiting method and limiter apparatus, and in particular to limiter apparatus for suppressing the peak factor of transmission power for multiple users in the CDMA technology is used as a multiple access system.
In a mobile radio communications system, especially in a cellular portable telephone system, a base station uses the multiple access scheme to access idle radio links for communications in order to simultaneously communicate with a plurality of mobile stations (communication end offices). The multiple access scheme is roughly divided into the Frequency Division Multiple Access (FDMA) system, Time Division Multiple Access (TDMA) scheme, and Code Division Multiple Access (CDMA) scheme. The base station assigns multiple access parameters to mobile stations, depending on the multiple access scheme of the radio communications system used.
While a plurality of channels are prepared with different frequencies in the FDMA scheme and with shifted time in the TDMA scheme, the CDMA scheme uses both frequency and time for multiple access and identifies channels via inherent codes sent over signals thus resulting in worse frequency use efficiency and requiring broadband circuits. However, the CDMA scheme is characterized in that the scheme ensures a high privacy because signals are converted by overlaying codes and thus the CDMA scheme enjoys a widespread use.
In the CDMA scheme, a large current may flow instantaneously especially during multiple processing. Thus the so-called limiter feature for limiting transmission power is very important.
Conventionally, as shown in FIG. 11, limiter apparatus used in the CDMA scheme is composed of a comparator for comparing a limit value and transmission data and a suppressor for suppressing the limit value in case the transmission data is larger than the limit value.
In this apparatus, as shown in FIG. 12, each time input signals comprising I channel and Q channel are input as parallel data, the input data is compared with the limit value. Send data not to be limited is passed through the suppressor and transmission data to be limited is individually limited.
In such conventional limiter apparatus, there was a problem that transmission data was individually limited and the peak factor could not be suppressed. In case phase-rotated transmission data is limited, the maximum power of the transmission data may drop by as much as 3 dB due to constraints on the input dynamic range of a D/A converter, thus making it difficult to optimize the number of users and transmission power.
As another example, a limiting circuit as shown in FIG. 13 is proposed. This circuit is adapted to adjust gain via a variable gain multiplier 15 depending on the limit value preset by the limit value setting section 2 in case input signals comprising I channel and the Q channel are input as parallel data, as well as to multiply I channel and Q channel of signals which exceeded a value by the limit value respectively via a limiting circuit 16, and to output the resulting value as I channel and the Q channel.
The limiting circuit 16 is a limit operation unit which detects whether the limit value is exceeded or not for the I channel axis and the Q channel axis representing I channel and Q channel respectively and which has a limit range in the shape of an octagon as a vertical cross section. The limit area is shown in FIG. 14.
Operation of the limiter apparatus is described below.
When the input I channel and Q channel data is input as parallel data, the variable gain multiplier circuit 15 obtains the sum of the absolute values of I channel data and Q channel data and compares the sum of the absolute values with a limit value preset by the limit value setting section 2. In case the sum of the absolute values is above 21/2 times the limit value, signals obtained by multiplying the input I channel and Q channel signals by 21/2 are divided by the sum value and the resulting values are output as output signals. On the other hand, in case the sum of the absolute values is equal to or below 21/2 times the limit value, signals are output without multiplication, so that the absolute values of the input I channel signal and Q channel signal converted by the variable gain multiplier circuit will not exceed the predetermined limit value. The limit operation unit is described in for example Japanese Patent Publication No. 5-328776.
Because, in the conventional method, limiting operation was made on I channel and Q channel transmission data individually, there was a problem that a larger limit value led to insufficient suppression effects. Further, when I channel and Q channel transmission data which were phase-rotated, the maximum power of the I channel and Q channel transmission data was excessively suppressed due to constraints on the input dynamic range of the D/A converter which received limiter apparatus output.
The object of the invention proposed in view of the circumstances is to enable appropriate power control and to optimize transmission power.
In order to attain this object, in the first aspect of the invention, a limiting method comprises a step of: limiting signals having two components I channel and Q channel on two orthogonal coordinate axes within a predetermined range on the coordinate plane specified by the two coordinate axes, wherein the predetermined range is defined by concentric circles having the origin of the two coordinate axes as a center.
According to such a method, optimum power suppression is enabled without excessively suppressing the maximum power.
In the second aspect of the invention, a limiting method comprises steps of: calculating the instantaneous power of signals having two components I channel and Q channel on two orthogonal coordinate axes of data to be sent; determining whether the instantaneous power is within a predetermined range or not; and a step for shifting the instantaneous power in the direction of the origin on straight lines connecting the origin and the signal component coordinates, in case the instantaneous power is determined to exceed the predetermined range.
According to such a configuration, optimization of power is possible with ease and efficiency.
In the third aspect of the invention, limiter apparatus comprises: an instantaneous power calculator for calculating an instantaneous power value from transmission data; a limit value setting section for setting a limit value; a comparator for comparing the instantaneous power value calculated by the instantaneous power calculator with the limit value; a correction determining section for determining whether transmission data is to be corrected or not depending on the comparison result of the instantaneous power value and the limit value; and a correction operating section for correcting the transmission data based on the correction value determined by the correction determining section.
According to such apparatus, power can be optimized with excellent efficiency.
In the fourth aspect of the invention, limiter apparatus comprises: an instantaneous power calculator for calculating an instantaneous power value from parallel input data composed of I channel and Q channel; a limit value setting section for setting a limit value; a divider-comparator for dividing the instantaneous power value calculated by the instantaneous power calculator by the limit value set by the limit value setting section to compare the instantaneous power with the limit value; a correction determining section for determining whether the input transmission data is to be corrected or not according to the comparison result; and a correction operating section for correcting the transmission data based on the correction value determined by the correction determining section and the input value.
In addition to the aforementioned effects, a correction value can be calculated via division according to such apparatus. This enables high-accuracy and reliable correction.
In the fifth aspect of the invention, limiter apparatus according to the third aspect of the invention wherein the comparator includes a subtracter-comparator for subtracting the limit value set by the limit value setting section from the instantaneous power value result calculated by the instantaneous power calculator to compare the instantaneous power with the limit value, wherein the correction determining section is adapted to determine whether input data is to be corrected or not depending on the comparison result of the subtracter-comparator.
According to such a configuration, a correction value can be calculated via subtraction alone, without using division, thus simplifying the apparatus configuration.
In the sixth aspect of the invention, limiter apparatus according to the third aspect of the invention wherein the instantaneous power calculator calculates the square value of parallel input I channel and Q channel transmission data without performing root operation of the data, wherein the comparator includes a subtracter-comparator for subtracting the limit value set by the limit value setting section from the instantaneous power value calculated by the instantaneous power calculator to compare the square value of the instantaneous power with the limit value.
According to such apparatus, operation of transmission data is possible without root operation, thus simplifying the operation process.
In the seventh aspect of the invention, limiter apparatus according to the third aspect of the invention further comprises a correction selector for outputting the parallel input I channel and Q channel transmission data without correction, in case correction is not required depending on the result of the correction determining section.
According to such apparatus, high-speed processing can be applied to non-corrected data. Moreover, power consumption can be reduced.
In the eighth aspect of the invention, limiter apparatus comprises: an instantaneous power averaging section for calculating an instantaneous power value from parallel input I channel and Q channel transmission data and obtaining the average value of instantaneous power for a certain period; a limit value setting section for setting a limit value; a comparator for comparing the instantaneous power average value obtained by the instantaneous power averaging section with the limit value; a correction determining section for determining whether the transmission data is to be corrected or not depending on the comparison of the average value of the instantaneous power with the limit value; and a correction operating section for correcting the transmission data based on the correction value determined by the correction determining section.
According to such a configuration, correction determination is made via the average value of instantaneous power, thus allowing a more stable control.
In the ninth aspect of the invention, limiter apparatus according to the eighth aspect of the invention wherein an instantaneous power calculator for calculates the square value of parallel input I channel and Q channel transmission data without performing root operation of the data, wherein the comparator comprises a subtracter-comparator for subtracting the limit value set by the limit value setting section from the instantaneous power value calculated by the instantaneous power calculator to compare the square value of the instantaneous power with the limit value.
In the tenth aspect of the invention, limiter apparatus according to the eighth aspect of the invention further comprises a correction selector for outputting the parallel input I channel and Q channel transmission data without correction, in case correction is not required depending on the result of the correction determining section.
In the eleventh aspect of the invention, limiter apparatus according to the third aspect of the invention further comprises a serial-to-parallel converter for converting transmission data composed of serial input I channel and Q channel to parallel data.
In the twelfth aspect of the invention, limiter apparatus comprises: an instantaneous power maximum value detector for detecting the maximum value of the instantaneous power for a certain period; a limit value setting section for setting a limit value; a comparator for comparing the instantaneous power average value obtained by the instantaneous power maximum value detector with the limit value; a correction determining section for determining whether the transmission data is to be corrected or not depending on the comparison result of the instantaneous power maximum value with the limit value; and a correction operating section for correcting the transmission data based on the correction value determined by the correction determining section.
In the thirteenth aspect of the invention, limiter apparatus according to the third aspect of the invention further comprises a feedback section which inputs as input data the sum of the correction value and the input data again to the instantaneous power calculator to repeat operation.
According to such a configuration, it is possible to perform a coarse operation using an approximate correction value in the first operation, and to perform fine control in the second operation. Performing more than one operation allows a higher-accuracy correction.